Employee Benefits - Narrative (Details)	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2016 benefit_plan	Dec. 31, 2009 benefit_plan
Defined Benefit Plan Disclosure [Line Items]
Pre-tax AOCI not yet recognized as net pension cost		$ 471,000,000	$ 511,000,000
Accumulated benefit obligation for all pension plans		$ 1,200,000,000	$ 1,200,000,000
Percentage increase or decrease in expected return on plan assets		0.25%
Estimated increase or decrease in net pension cost		$ 3,000,000
Percentage increase or decrease in assumed discount rate		0.25%
Estimated change in net pension cost due to discount rate		$ 2,000,000
Expected return on plan assets		4.50%	4.75%	6.00%
Expected return on plan assets on estimating 2013 pension cost		3.75%
Employer contribution to saving plan		6.00%
Annual increase to default employee contribution, percent		1.00%
Employer discretionary contribution, required service period		1 year
Employer discretionary contribution percentage		1.00%	2.00%	2.00%
Additional employer discretionary contribution amount, per full-time employee		$ 1,000
Additional employer discretionary contribution amount, per part-time employee		500
Additional employer discretionary contribution amount, salary requirement		100,000
Total expenses associated with saving plan		$ 98,000,000	$ 106,000,000	$ 129,000,000
Initial Default Percentage [Member]
Defined Benefit Plan Disclosure [Line Items]
Initial default employee contribution, percent		2.00%
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2020		$ 94,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2021		93,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2022		91,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2023		90,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2024		88,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2025 through 2029		394,000,000
Accumulated benefit obligation for all pension plans		1,054,000,000	1,029,000,000
Net pension cost (benefit)		$ 29,000,000	$ 22,000,000	$ (5,000,000)
Expected return on plan assets		4.50%	4.75%	6.00%
Discount rate		2.89%	4.00%
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pre-tax AOCI not yet recognized as net pension cost		$ (27,000,000)	$ (12,000,000)
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2020		7,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2021		7,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2022		6,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2023		5,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2024		5,000,000
Benefits expected to be paid from all funded and unfunded pension plans/other postretirement plans in 2025 through 2029		24,000,000
Net pension cost (benefit)		$ (1,000,000)	$ (1,000,000)	$ 1,000,000
Expected return on plan assets		4.50%	4.50%	4.50%
Age of employees under condition one		55 years
Period of service under condition one		5 years
Age of employees under condition two		50 years
Period of service under condition two		15 years
Minimum period of service at the time of termination hired before 2001		15 years
Discount rate		4.50%	4.00%
First Niagara Bank, N.A. [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of plans | benefit_plan					2	2
Forecast [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net pension cost (benefit)	$ 13,000,000
Forecast [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Net pension cost (benefit)	$ 0
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Employees contribution to saving plan		1.00%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Employees contribution to saving plan		100.00%
Default employee contribution, percent		10.00%